Tooth-colored dental bonded fillings that are placed in cavity preparations following caries (cavity) removal are a popular alternative to traditional silver fillings. Patients prefer these cosmetically attractive restorations but the procedure is technically more difficult than that for silver fillings.
The technique for the placement of tooth-colored dental bonded fillings may be divided into two types: First, the direct technique wherein the filling is a light-cured or chemically-cured bonding material that is directly shaped in the cavity preparation and hardened in place right after caries removal. The material is usually an acrylic based polymer. Second, the indirect technique where at the first visit, the dentist removes the cavity and takes an impression (or mold) of the tooth. The patient leaves with a temporary filling. At the second visit the dentist inserts a custom-fabricated dental filling that is then sealed in place with a light-cured or chemically-cured bonding material. The fabricated filling material can be acrylic, porcelain or xe2x80x9cceromerxe2x80x9d, a hybrid of an acrylic and a ceramic powder filler.
In the direct technique, unlike traditional silver fillings that are inserted in a cavity preparation as a semi-solid mass, the bonding material is a viscous liquid, which can flow in unintended places and encapsulate voids that compromise the strength of the restoration. The bonding material, usually an acrylic, is light-cured or chemically cured to harden after placement.
Another difficult part of the procedure for dentists is making the dental bonding material contact the adjacent tooth. This is necessary to prevent subsequent food impaction should a contact be left open. The tendency of the semi-cured material to stick to any instrument used to manipulate it results in xe2x80x9ctug backxe2x80x9d from the bonding surface, especially when bonding to moist dentin. The difficulty in creating a smooth surface transition between the natural and restored surface can act to trap bacterial debris.
The insertion of tooth-colored dental bonded fillings into a clean cavity preparation in a tooth requires the xe2x80x9cbuilding upxe2x80x9d of multiple layers, each with it""s curing cycle and potential for the formation of a delaminated interface between layers from potential wet films on the surfaces. The hardened underlayers have fewer chemical bonding sites for the next layer to adhere to requiring physical abrasion for proper bonding. Each subsequent layer must be thoroughly hardened before its surface can be abraded.
Polymerization shrinkage, which occurs while curing the directly placed bonding material can result in the withdrawal of the bonding compound from the walls of the cavity and causes tensile stresses between the bonded surfaces that compromise the life of the restoration.
In the indirect technique, a dental filling material is hardened in a dental laboratory using heat and pressure during curing. This has superior mechanical properties to one directly placed and hardened in a tooth. The customized, indirectly fabricated dental restoration is stronger and more wear resistant but it takes two visits and more of the dentist""s time. This is significantly more expensive and more inconvenient to the patient. It is also more expensive because of the cost of custom fabrication of the restoration by a dental laboratory. Furthermore, a tooth can become increasingly sensitive each time it is worked on and this technique requires at least two interventions and sometimes more if the custom filling does not fit. This disclosure involves a hybrid of the two techniques.
Copy milling or CAD-CAM milling (Computer Aided Design-Computer Aided Manufacturing) of blocks of ceramics or composites can produce indirect inlays and onlays chair side. One well-known Cad-Cam machine is made by Cerec (Siemens, Germany). This machine scans an optical impression of the tooth preparation following caries removal and mills an inlay or onlay out of a block of ceramic or composite in only a few minutes. The disadvantages of this technique include the need for significant training and the inaccuracy of the fit of the inlay or onlay. Currently, these machines cost $90,000 to purchase plus $40 for each ceramic blank, which makes it very expensive for patients. In contrast, prefabricated dental inlay preforms are intuitive to use and don""t require any special machinery so the cost per use would only be a few dollars at most. The accuracy of the fit of the preforms, like Cerec, will not match that provided by a custom lab fabricated dental inlay or onlay, but it will be a lot faster, more convenient and less invasive to the tooth. It will also be a lot stronger than the direct technique. Unlike Cerec and custom inlays or onlays, it will be very cost-effective for patients.
The Invention
A method is described to help dentists overcome these difficulties with the use of prefabricated, geometrically shaped, inlay preforms of various sizes with approximately trapezoidal, rectangular and square faces. The cavity preparation of a tooth is roughly one of these shapes. It is possible for a dentist to choose from assorted sizes of prefabricated, geometrically shaped dental inlay preforms to see which one might most ideally fill in the tooth. Minor misfits could be removed with a dental drill on internal surfaces. The practitioner would place the chosen piece, or pieces, into the space of the cavity preparation after first placing a layer of chemically cured, flowable bonding material. This latter material, that is not a unique part of this disclosure, would function in a manner similar to cement encompassing a floor tile. The end result is a restored tooth with a smooth, continuous exterior almost indistinguishable in appearance and strength from the original tooth.
The prefabricated dental inlay preforms are made in laboratory conditions using heat and pressure during curing to provide superior mechanical properties. The applicant""s preforms could be manufactured using thermally cross-linked acrylic polymers, tough ceramic porcelains or hybrid xe2x80x9cceromersxe2x80x9d. Kits of prefabricated shapes of enough variety to anticipate any needed restoration are contemplated by this invention. Minor chair-side modifications will tailor the prefab shape to the individual tooth. The occlusal and proximal surfaces can be finished to match the adjoining teeth. Applicant""s preforms can be equipped with sidewall grooves to offer an interlocking means or roughened or sand blasted surface to add more bonding sites. This insures that these preformed, polymerized inserts have better mechanical bonding characteristics. Small spacing bumps, hemispheres or conic projections, molded onto the bonding surfaces of the preforms will space the preform a known distance from the surfaces of the cavity excavation to insure an adequate thickness of bonding compound within the bonding interface.
Applicant""s invention provides patients with the strength of an indirectly fabricated tooth-colored filling that is placed with the one-visit convenience and cost savings of a direct filling. There will be less wear on the biting surface of a restoration manufactured extra-orally under controlled pressure and temperature. Since the bulk of the filling will already be completely polymerized during manufacture there will be much less shrinkage during the insertion and hardening of the filling in the tooth. The use of a preform results in a finer gap junction with a smaller bonding interface inducing less of the shrinkage stresses, maintaining the hybrid adhesive junction between the sealed surfaces and decreasing the potential for microleakage.
Applicant envisions a few dozen shapes mainly in the molar and premolar sizes for the upper and lower arch of teeth. Various colors of the preform could also be offered. For simplicity it is thought to initially offer a shade comparable to A2 on the Lumin (Tm) scale. A small tab or handle temporarily attached to the biting surface of the preform will ease its manipulation when trying it in or finish-cutting it. This can be cut away after setting in place or just prior to final bonding.
Though hybrid technologies are used to form full veneer temporary crowns, none have been considered for cavity filling preforms envisioned by the applicant. The shapes contemplated are singly or in combination used to fill the typical void formed in decay removal in the intracoronal and proximal areas of mainly molars and premolars. The inlay perform shapes will closely match the standard cavity preparation classification system as described in Sturdevant""s xe2x80x9cThe Art and Science of Operative Dentistry, Fourth Edition. Copyright 2002.xe2x80x9d These Classes are labeled with Roman numerals I through V. Class I is a repair to the occlusal surface alone. Class II involves two or more tooth surfaces in molars and premolars, such as the mesioocclusal (MO) or the distoocclusal (DO) surface. Reconstruction to three surfaces in the mesioocclusodistal (MOD) is considered repairable by all the methods discussed. Repairs involving the complete replacement of a portion of the crown of the tooth including at least one cusp are considered to be partial onlays and not defined as an inlay for the purpose of this invention. Complete crown replacement is beyond the techniques described in this application. Other classes involving repairs to facioocclusal and linguooclusal excavations. Partial anterior tooth surfaces also can be within the scope of this invention.
There are several commercial advantages to this invention. This invention incorporates the strength of a prefabricated dental inlay shape with the efficiency of a direct bonded filling. A stronger filling can be placed in teeth than that obtained with the most commonly used direct bonding technique. This indirectly fabricated inlay could be placed into a tooth in one visit, instead of two, saving the patient a lot of time and substantially reducing the likelihood of a root canal flare-up. This procedure could be priced more like that of a typical direct bonded filling since it only requires one visit and there is no additional laboratory fee associated with custom fabrication. It could dramatically alter the way restorative dentistry is practiced.